In a substrate processing apparatus configured to perform a preset plasma process on a wafer as a substrate by using plasma generated within a processing chamber, components (parts) provided within the processing chamber may be worn out by the plasma. Especially, since a focus ring provided to surround the wafer and made of the same material as the wafer is exposed to the plasma having a relatively high density, a wear rate of the focus ring may be high. If the focus ring is worn out, plasma distribution on the wafer would be varied. Thus, the wear rate of the focus ring needs to be monitored, and the focus ring should be replaced if its wear rate exceeds a preset level.
Conventionally, the wear rate of the focus ring has been measured at the outside of the processing chamber after the focus ring is taken out of the processing chamber. In order to take the focus ring out of the processing chamber, however, the processing chamber needs to be opened to the atmosphere. After the focus ring of which a wear rate has been measured is returned back into the processing chamber, it takes time to evacuate the processing chamber again. As a result, an operating rate of the substrate processing apparatus is greatly deteriorated.
To solve the problem, the present inventor has proposed a method for measuring the wear rate of the focus ring by measuring a thickness of the focus ring through a process of irradiating a low-coherence light beam toward a bottom surface of the focus ring within the processing chamber and calculating a variation of the thickness (i.e., the wear rate of the focus ring) from the measured thickness (see, for example, Patent Document 1). In this method, an optical path length of the low-coherence light beam that travels forward and backward within the focus ring in a thickness direction thereof is calculated based on an interference between a reference light beam and a low-coherence light beam reflected from the bottom surface of the focus ring and an interference between the reference light beam and a low-coherence light beam reflected from a top surface of the focus ring. Then, the thickness of the focus ring and the wear rate of the focus ring are measured based on the calculated optical path length.
In accordance with the aforementioned method, since it is possible to measure the wear rate of the focus ring without taking the focus ring out of the processing chamber, a great reduction in the operating rate of the substrate processing apparatus can be prevented.
Patent Document 1: Japanese Patent Laid-open Publication No. 2008-227063
However, the focus ring is made of silicon and, in general, the silicon thermally expands and its refractive index varies depending on a temperature. Thus, if a temperature of the focus ring increases during a plasma process, the optical path length of the low-coherence light beam would be also increased. Accordingly, if the temperature of the focus ring varies, the wear rate of the focus ring cannot be measured accurately by the method of using the interference of the low-coherence light beam. Thus, only when the temperature of the focus ring is maintained at a preset temperature, the wear rate of the focus ring can be measured by this method. By way of example, only when the substrate processing apparatus is in an idle state, the wear rate of the focus ring can be measured by this method. Thus, a chance to measure the wear rate of the focus ring may be limited if this method is used.